しとけよ (colloquial) = get ready, prepare yourself! Verb-te + oku -> Verb-toku: "do something in preparation" しとく+ potential form (く => け) + imperative (よ)

It's the colloquial way of しておく. The closest expression is something like 'have it done'. See the examples bellow. Set your mind x Have your mind set. While the former sounds urgent, the latter sounds doing so for some occasion coming. In this way, しておく eludes the state of preparation.